30S-1 Linear Amplifier

THE COLLINS 30S-1 LINEAR AMPLIFIER

Add the Collins 30S-1 Linear Amplifier to your station and
you will have the cleanest, loudest signal on the air. The
Collins 30S-1 Linear Amplifier features:

RF INVERSE FEEDBACK for better linearity.

INSTANT SWITCHING between low and full kilowatt power.

QUICK AND ACCURATE TUNING, offering the amateur a bonus in
ease of operation and optimum operating efficiency.

AUTOMATIC LOAD CONTROL, assuring a clean signal.

COOLING SYSTEM which operates quietly and efficiently.

SIMPLE AND DIRECT CONNECTION between the 30S-1 and the
exciter and station control unit.

PROTECTIVE CIRCUITRY which protects tubes and other
components from damage due to mistuning or malfunction.

Requiring 70-100 watts driving power (supplied by the
Collins 32S-3 Transmitter or KWM-2 Transceiver) the 30S-1
Linear Amplifier provides your SSB and CW station with the
full legal power input for SSB (1 kw average) or 1 kw input
for CW transmission.

All the 30S-1 controls are easily accessible on the front
panel. This front panel design allows you to tune the 30S-1
swiftly, surely and easily. With the push of a button you
can switch instantly from the 100 watt power level of your
S/Line transmitter to the full kilowatt output of the 30S-1;
yet you retain high linearity and clean signal. The 30S-1
can also be tuned to frequencies outside of the amateur
bands. Automatic Load Control voltage from the 30S-1 is fed
back to the transmitter, assuring you of maximum talking
power without over driving and distortion. Collins Automatic
Load Control, in combination with Collins' RF inverse
feedback, is a major design feature in the 30S-I which gives
you more talking power with a cleaner signal than any other
linear amplifier in the amateur field.

The 30S-1 is a completely self-contained, single tube,
grounded grid linear amplifier. The tube used is the
commercially popular Eimac 4CX1000A.

Correct tuning and loading are indicated by a zero reading
on a full scale multimeter. The loading control and PA
tuning control are simply adjusted to obtain zero meter
indication.

Under the hood...

30S-1 CIRCUITRY

The power amplifier stage is a single, ceramic tetrode which
is cathode driven. The grid is grounded for r-f by capacitor
C104. The screen grid is connected directly to ground. The
plate power supply, the screen grid power supply, and the
control grid bias supply are connected in series. The
junction between the plate power supply and the screen grid
power supply is grounded through the screen current meter
shunt. This arrangement places the cathode at negative
potential with respect to the screen grid. The bias supply
is connected between the cathode and the control grid.
Provisions are included for r-f negative feedback to improve
linearity and for automatic load control (alc) to prevent
overdrive.

INPUT CIRCUITS

Pi-network broad-tuned circuits and the interconnecting r-f
feed line match the 50-ohm input impedance to the cathode
impedance, which is approximately 100 ohms. The 20.5-foot
length of cable (furnished) is necessary between the 32S-1
(or KWM-2) driver and the 30S-1 input circuits. This is due to
the necessity of having an even multiple of 180-degree phase
shifts between driver plate and power amplifier grid. The
cable length and the 30S-1 input circuits together
accomplish this. An even multiple of 180-degree phase shifts
is necessary because modulation components cause a change in
the resistive PA cathode impedance which is translated to a
shift in reactive impedance at the driver plate. The shift
in reactive impedance, at the driver plate, results in phase
modulation of the driver and increases the total over-all
distortion of the system. A 2.5-foot additional length of
cable is furnished to bring the total interconnecting cable
length to 23.0 feet for use with the KWM-1 as a driver.
Drive power required for maximum legal input on SSB is 80
watts PEP.

OUTPUT CIRCUITS

The plate circuit of the power amplifier is tuned by a pi
network consisting of C120, L109, L104, C121, and C122.
Capacitors C121 and C122 are ganged together and are
adjustable by front panel control (LOADING) for matching the
pi-network circuit to the impedance of the antenna and feed
system in use. Capacitor C120 may be adjusted by the TUNING
control on the front panel for resonating the tank circuit
to the frequency in use. Output from the plate tank circuit
is connected through the contacts of antenna changeover
relay K101 to the antenna when the control circuits are
switched to transmit function.

POWER SUPPLY CIRCUITS

Three d-c power supplies and three a-c filament supplies are
included in the 30S-1. The power supply may be connected to
115-volt single-phase or to a 230-volt, three-wire,single-
phase source. The 230-volt, three-wire connection is
recommended. High-voltage plate transformer T201 has two
primary windings. These windings are connected in parallel
for 115-volt operation, and in series for 230-volt operation.
The 12-volt a-c filament winding of the bias supply
transformer supplies current for the filament of the alc
rectifier, the pilot lamps in the two meters, and the pilot
lamps which light the two dials. This transformer winding
also supplies current for rectified d-c relay power. The
bias winding of the transformer, T203, is connected to CR207
and CR208 in a full-wave rectifier circuit. This circuit
provides grid bias voltage for the power amplifier. The
heater of the 3-minute time-delay relay is supplied power
from the 115-volt a-c connections which also furnish power
to the high-voltage rectifier filament transformer, T202.
The filament transformer, T103, supplies a-c power for the
heater of the thermal over-load relay, K102. Taps on the
primary of the high-voltage plate transformer, T201, are
switched to provide the different voltages necessary for the
power amplifier in CW or SSB operation. Power amplifier bias
voltage is switched to one of two taps on the bias supply
bleeder resistors for CW or SSB operation. The high-voltage
screen supply rectifiers are eight, type 1N1492 silicon
diodes in a full-wave bridge circuit. Each rectifier diode
is paralleled with a 0.001-uf capacitor to protect it
against high transient voltages. The high-voltage plate
supply has a secondary winding from T201 and its high voltage
is rectified by a pair of 3B28 rectifier tubes V201 and V202.

SAFETY INTERLOCK CIRCUITS

The top cover and the power supply front door operate safety
interlock switches for operator protection. When the top
cover is opened, interlock switch S103 breaks the circuit to
the coil of the plate contactor, K203. This removes all high
voltages from the 30S-1. When the power supply compartment
(lower) door is opened, interlock switch S205 breaks the same
circuit and removes all high voltages. Both interlock
switches are mechanically interlocked with shorting switches
which short out the high-voltage filter capacitors at the
same time the interlock circuit opens. The r-f compartment
interlock switch, S102, is mechanically ganged with shorting
switch S101, and the power supply compartment interlock
switch, S205, is mechanically ganged with shorting switch
S206. This arrangement protects the operator from
accidentally coming in contact with approximately 3000 volts
d-c which is present in either compartment.

TIME DELAY AND STEP-START CIRCUITS

When POWER-OFF switch S202 is closed, the 115-volt a-c power
is applied to the heater of the 3-minute time-delay relay,
K202. After the power has been applied to its heater for
approximately three minutes, the bimetallic contacts close.
These contacts are in series with the interlock circuits and
the coil of plate contactor K202. When the ON push button is
depressed, K202 is energized, contacts of K202 close and
apply power to the step-start relay, K201, through d-c
rectifier CR205. The large electrolytic capacitor, across
the coil terminals of K201, requires a fixed charging time
to rise to a potential high enough to energize the relay.
When this time has passed, K201 energizes and shorts out the
step-start resistors. Until relay K201 has closed, all power
applied to the transformer primary winding has been dropped
through the two step-start resistors, R201 and R233. Thus,
the high-voltage power supply starts at low primary voltage
and, after the step-start cycle has elapsed, switches to
full voltage. This allows time for partial charging of the
large, high-voltage filter capacitors, C207 and C208, before
the application of full secondary voltage to the rectifier
plates. During this time, the rectifier tubes are protected
from damaging high peak currents.

THERMAL AND OVERLOAD CIRCUITS

The thermal overload relay, K102, protects the power
amplifier tube from over dissipation and loss of cooling air.
Its bimetallic strip has contacts connected in series with
the interlock system. The thermal overload switch is located
in the air stream from V101. Current from transformer T103
is passed through the heater of K102. This current keeps K102
temperature just below that necessary to open its contacts.
If the air stream fails, the temperature of the bimetallic
strip increases, opening the interlock circuit, and removing
voltages from the power amplifier. If over-dissipation
occurs in the plate of the power amplifier, the higher air
temperature causes K102 to operate and break the interlock
circuit.

POWER CONTROL CIRCUITS

When the POWER-OFF switch is operated to POWER position,
115-volt a-c power is applied to the filament and control
circuits. If the 3-minute time delay of K202 has passed, and
if all interlock circuits are in proper operating condition,
the plate contactor may be energized by pushing ON switch
S203. When K203 contacts close, one set of them holds the
electrical connection to the coil and keeps the relay closed
after the ON push button is released. Other contacts of K203
supply power to the antenna changeover relay circuit and to
the primary winding of the high-voltage transformer, T201.
Power to T201 is applied from K203 contacts through two
step-start resistors . These resistors reduce the voltage
applied to the transformer until capacitor C203 charges high
enough that the voltage across it will energize step-start
relay K201. When K201 closes, its contacts short out the
step-start resistors and allow full voltage to be applied to
the transformer winding.

ALC AND R-F NEGATIVE FEEDBACK CIRCUITS

Automatic load control (alc) is a type of compressor circuit,
operating at radio frequencies. The modulation envelope is
detected by power amplifier grid rectification. This signal
is filtered of r-f by L108 and C140 and applied through
transformer T102 to the alc rectifier, V203. The audio signal
is rectified by V203 to produce a negative control voltage
which is a function of the modulation level. The alc
rectifier, V203, is connected as a voltage doubler. The
negative control voltage produced by the alc rectifier is
fed back to the alc line of the exciter to produce
approximately 3 db of override control. The resistor, R234,
in series with V203 filament, reduces the no-signal d-c
level on the alc line. This no-signal d-c level is caused by
the tube contact potential. If not reduced, it might cause a
delay voltage to be present on the exciter alc bus.

The 3 db of alc override control produced in the 30S-1 reduces
the exciter r-f gain and keeps the drive level within
tolerable limits. Automatic load control helps to keep the
drive level low enough to prevent driving the power
amplifier into distortion.

A fixed amount of r-f negative feedback, from the output
circuit of the power amplifier to the input of the power
amplifier, produces a high degree of linearity of the
amplified signal. This feedback is accomplished by
capacitor C103, which couples some of the plate energy back
to the grid circuit. Although there is no phase inversion
between the cathode and the plate circuits of a cathode-
driven amplifier, there is a phase inversion between the cathode
and the grid circuit, providing the grid is not bypassed
completely at the r-f frequency. Therefore, the feedback
voltage is out of phase with the grid voltage. Capacitors
C103 and C104 form a voltage divider circuit to maintain the
proper amount of feedback voltage.

TUNING & LOADING METER CIRCUIT

One section of the SSB-CW switch, S201, selects the proper
output voltage from the tuning and loading bridge circuit
for the TUNING & LOADING meter indication. This circuit and
the power amplifier tube form a specialized vacuum-tube
voltmeter bridge circuit. It consists of V101, CR101A, and
CR101B, and the associated load resistors and filter
networks. The bridge is balanced when the plate circuit
TUNING and LOADING controls are set to present the proper
load impedance to the power amplifier plate. The meter then
will read zero, and the power amplifier tube will be
operating at the proper gain level for maximum efficiency
and linearity.